In conjunction with air conditioning, the following issues must be considered: recovery of exhaust air energy, moisture balancing and cooling/heating. In addition, these matters have a different impact in terms of geography. In cold zones, heat recovery is vital, in the temperate zone cooling and moisture balancing are also primary issues, whereas the focus is on cooling and dehumidification of outdoor air in the hot zone.
It has been stated that cooling entails costs that are about six times those of heating. The cooling of outdoor air into indoor air requires much energy in the warm zone. Due to the high connection power required by cooling device compressors, operating such devices with the use on renewable forms of energy is not justified.
In the summer, the relative humidity of indoor premises tends to be too high, whereas it is too low in the heating season. This is due to the fact that hot air may contain more humidity. For the moisture balance to be ideal, expensive air dryers and humidifiers are needed.
Condensed humidity releases heat, and similarly, evaporated humidity binds heat. By making skilful benefit of these natural phenomena, the relative humidity of indoor air can be kept in balance and energy consumption reduced.
Air dehumidification is more difficult and expensive than mere cooling. As a solution to this, it has even been proposed to recirculate indoor air that has been dried once, however, this involves the problem of lack of fresh air. Humans, animals and buildings themselves are in need of fresh air. The reason for such solutions is the high energy consumption caused by conventional techniques.
Besides humidity problems, moisture condensation in heat recovery cells results in the cell array freezing at temperatures below zero without additional heating of the cells. In conventional plate heat exchangers on bakeries, humidity associated with flour and yeast dust will form a “dough” in the cells, so that air cannot pass through these. Then the heat from the ovens cannot be recovered, for instance.
Heated air may contain more humidity than cold air. Heated and moist air that has been conducted to a duct system may result in mildew damages, because mildew growth requires nothing but moisture and heat.
Impurities in indoor air cause serious sanitary and economic damage, let alone problems related to comfort. Humans, animals and plants all suffer from these problems. Outdoor air may be purer than indoor air, however, in cities and specifically in industrial environments the outdoor air is more polluted than the indoor air.
If the filters are not replaced according to instructions, the impurities will increase. If the location of a filter is sufficiently hot and humid, there will be bacterial growth on its surface, even of lethal legionella in the extreme.
Air contains particles differing in size/diameter from those that can be seen with the bare eye—a diameter above 5 micrometers, visible only with a special microscope a diameter p less than 0.1 micrometer. The greatest impurities consist of e.g. hair, pollen and coal dust. Smaller impurities are e.g. bacteria, dust which is hazardous for the lungs, sulphur compound fog, oil mist and lead monoxide. The smallest particles consist of viruses and fumes.
Air filters can be roughly divided into coarse filters, which clean impurities that can be seen with the bare eye, fine and electric filters, which clean also impurities visible only with a microscope, and finally hepa filters, which clean practically all impurities.